Mammalian cell membranes are important to the structural integrity and activity of many cells and tissues. Of particular interest in membrane physiology is the study of trans-membrane ion channels which act to directly control a variety of pharmacological, physiological, and cellular processes. Numerous ion channels have been identified including calcium, sodium, and potassium channels, each of which has been investigated to determine their roles in vertebrate and insect cells.
Because of its involvement in maintaining normal cellular homeostasis, much attention has been given to potassium channels. A number of these potassium channels open in response to changes in the cell membrane potential. Many voltage-gated potassium channels have been identified are characterized by their electrophysiological and pharmacological properties. Potassium currents are more diverse than sodium or calcium currents and are further involved in determining the response of a cell to external stimulus. The diversity of potassium channels and their important physiological role highlights their potential as targets for developing therapeutic agents for various diseases.
One of the best characterized classes of potassium channels are the voltage-gated potassium channels. The prototypical member of this class is the protein encoded by the Shaker gene in Drosophila melanogaster. In addition to Shaker, three other classes of potassium channels. Shap, Shaw, and Shal have been identified in Drosophila Proteins of the Shal or the mammalian Kv4 family are a type of voltage-gated potassium channels that underlies many of the native A type currents that have been recorded from different primary cells (Serodio et al. 1994, 1998; Dixon et al. 1996). The Kv4 channel has a major role in the repolarization of cardiac atrial action potentials (Dixon et al. 1996). In neurons, Kv4 channels and the A currents they may comprise play an important role in modulation of firing rate, action potential initiation and in shaping burst pattern (Byrne 1980; Connor and Stevens 1971; Getting 1983; Hille 1992; Llinas 1988; McCormick and Huguenard 1992; Rudy 1988; Thompson and Aldrich 1980). Recently, A currents were localized to dendritic regions in the hippocampus where they play a complex role in modulating synaptic input and retrograde propagation of action potentials (Hoffman et al. 1997).
The gene encoding rat Kv4.3 channels have been cloned and expressed by several labs (Dixon et al. 1996; Serodio et al. 1996; Ohya et al. 1997 and Takimoto et al. 1997). The identification of the Kv4 family as the molecular correlate for the transient outward potassium current (also called the A current) in both heart and brain comes primarily from data obtained with Xenopus oocytes expressing rat Kv4 cloned channels. As in native neurons and myocytes, Kv4 channels display fast and complete inactivation at positive potentials. Initially however, the voltage dependency for steady-state activation and inactivation of these expressed Kv4 channels appeared to be shifted 20 mV positive to that seen in neurons, which would place them out of the subthreshold category. In addition the recovery from inactivation of the expressed channels was slower than that seen in the native cells. Fortunately it was discovered that addition of a low molecular weight RNA fraction from rat brain, presumably containing a beta subunit for the Kv4 channel, shifted the voltage dependence and speeds the channels rate of recovery from inactivation to that seen in the native cells (Serodio et al. 1994, 1996 and Chabala et al. 1993). Further confirmation of the role of Kv4 channels in forming A type currents comes from antisense hybrid-arrest and dominant-negative experiments in both neurons and cardiac myocytes (Johns et al. 1997; Fiset et al. 1997; Nakamura et al. 1997).
Until the present invention, the gene encoding human Kv4.3 had not been identified. The art needs molecular characterization of the human Kv4.3 channel in order to elucidate their function and properties in preventing or treating dysfunctions and diseases.